A number of unique refrigeration cycles and apparatus have been developed to satisfy the increasing demand for highly reliable, long-lasting cryogenic refrigerators for use in such diverse fields as electronic communications systems, missile tracking systems, super conducting circuitry, high field strength magnets, and medical and biology laboratories for preparation of tissue samples and freezing of solutions. These refrigeration cycles and apparatus, all based upon the controlled cycling of an expansible fluid with suitable heat exchange to obtain refrigeration, are exemplified by U.S. Pat. Nos. 2,906,101, 2,966,034, 2,966,035, 3,045,436, 3,115,015, 3,115,016, 3,119,237, 3,148,512, 3,188,819, 3,188,820, 3,188,821, 3,218,815, 3,333,433, 3,274,786, 3,321,926, 3,625,015, 3,733,837, 3,884,259, 4,078,389, and 4,118,943, and the prior art cited in the foregoing patents.
The present invention is directed at refrigeration systems which employ a working volume defined by a vessel having a displacer therein with a regenerator coupled between opposite ends of the vessel so that when the displacer is moved toward one end of the vessel, refrigerant fluid therein is driven through the regenerator to the opposite end of the vessel. Such systems may take various forms and employ various cycles, including the well known Gifford-McMahon, Taylor, Solvay and Split Stirling cycles. These refrigeration cycles and apparatus require valves or pistons for controlling the flow and movement of working fluid and the movement of the displacer means. The fluid flow and the displacer movement must be controlled continuously and accurately so that the system can operate according to a predetermined timing sequence as required by the particular refrigeration cycle for which the system is designed. Although a fixed timing sequence is the usual objective, it also is desirable to be able to alter the sequence in certain respects, e.g., the time over which high pressure fluid is introduced to the vessel or the time period during which expansion and cooling are achieved.
Heretofore the valving of cryogenic equipment of the type described has taken various forms, but inevitably the valving or the resulting refrigerator has suffered from one or more of the following limitations: complexity of construction, relatively high cost of manufacture, difficulty of modification as to timing sequence, relatively short operating life, poor reliability, difficulty of adjustment after assembly, and small range of refrigeration capacities. Additional specific problems that have plagued some prior cryogenic equipment have been disintegration of lead shot in the regenerator section due to the "slamming" or "banging" of the displacer on its mechanical stops each time it undergoes direction reversal, excessive size of the valving (or of the refrigerator because of the valving construction and/or location), the criticality or short life of seals between certain moving parts, reduced efficiency due to excessive work input or work absorption (e.g. high friction losses), and inability to operate at the low reciprocating speeds that are preferred for such apparatus. Added cost and performance limitations are presented by those devices where the diplacer movement is produced by mechanical means such as cams, eccentrics, etc.